A catamaran type of raft equipped with a hydraulic winch was constructed. Frames with mussels could be lowered and raised separately up to 3m above the sea surface in order to destroy predators and fouling organisms. The total capacity of the raft was 15t of mussels. Toxic mussels which were lowered in June to a depth of 30m, in a water layer with a low concentration of toxic algae, remained toxic until December. The meat content of these mussels decreased gradually during this period, but increased when lifted to the surface water. It is concluded that due to low temperature and poor nutrient conditions, ie low phytoplankton concentrations, below the pycnocline, the activity of the mussels was low. They retained the toxins as a result of the low metabolic rate.